1. Field of the Invention
The present invention relates to a laser diode (hereafter denoted as LD) driver, in particular, the invention relates to a laser driver with a shunt mode and driven with signals complementary to each other.
2. Related Prior Art
A United States Patent, the U.S. Pat. No. 6,618,408, has disclosed a laser driver with the shunt mode. The shunt driver includes a transistor connected in parallel to the LD. The gate electrode of the transistor receives the modulating signal, the source electrode being grounded common to the cathode of the LD, and the drain electrode is connected with the anode of the LD. Thus, the transistor may shunt the current externally supplied to the LD by the modulating signal applied to the gate electrode thereof.
When we set the shunt transistor to be an n-channel enhancement FET (hereafter denoted as E-FET), which often shows an effective cost, a positive DC bias of 0.7-1.0 V is necessary to be applied to the gate electrode of the transistor. While, a terminator is necessary to be coupled with the input terminal to reduce the signal reflection at the input terminal in high frequencies. When the input terminal is offset in the DC mode, this DC offset voltage results in the DC current flowing in the terminator directly connected to the ground, which increases ineffective power consumption.
Although a terminating capacitor may isolate the input terminal in the DC mode but ground in the AC mode, the capacitance thereof is necessary to be large enough so as not to affect the frequency response of the terminator in low frequencies. Such a capacitor with larger capacitance is substantially impossible to be integrated with in an IC, and inevitably set in the outside of the IC. A bonding wire may electrically connect such capacitor with the IC, but degrades the high frequency response of the terminator due to the parasitic inductance inherently attributed with the bonding wire.